1. FIELD OF THE INVENTION
The invention relates to molding apparatus for the hot pressing of parts made of a refractory material.
Known processes for shaping parts made of refractory materials, especially composite materials, and in particular parts made of ceramic materials intended for use in aircraft engines, generally involve a hot pressing operation which can be carried out in a press furnace at high temperature. Various heating means may be used for these furnaces, whether for radiant heating, or for induction heating, possibly with a susceptor, the heat being propagated from the outside towards the center.
2. Description of the Prior Art
Traditionally, the pressing assembly proper is realized in accordance with a so-called monobloc concept, in which the blank of the part to be obtained is placed between two dies which, for applications requiring temperatures ranging, for example, between 1200.degree. C. and 2200.degree. C. and the enclosure to be placed in a vacuum or a neutral atmosphere, are made of graphite. However, during high temperature heating tests (at 1400.degree. C. for example) on refractory materials, it is possible to observe differences of temperature in excess of 200.degree. C. between the outer edge and the center of the part. This radial thermal gradient is generally unacceptable for the quality of the parts to be obtained. Attempting to obtain temperature uniformity by insulation leads to excessively prolonging the duration of the cycle and, anyway, makes it impossible to achieve the necessary cooling rates after pressing, which in certain cases may reach some 1200.degree. C. per hour in order to obtain conditions equivalent to oil hardening.
Complying with these cooling rates leads to providing a separate mold which receives the blank and which permits pressing between the dies of the press-furnace as described above. In this case, the mold containing the pressed material may be removed from the furnace and rapidly cooled.
The techniques now used, for example, for shaping parts made of composite ceramic materials of the type comprising SiC fibres in a glass matrix and for the associated temperature ranges and conditions of operation (enclosure in a vacuum or neutral atmosphere), comprise the use of dies and a separate mold of graphite.
However, tests have shown that the thermal gradients, particularly in a radial direction, between an outer edge and the center, remain too high and incompatible with satisfactory implementation corresponding to the quality criteria of the results to be obtained.
French Specification No. 2 532 585 discloses an example of controlling the temperatures of a mold used for hot pressing with a view to obtaining uniformity of the mold temperatures. However, this solution, which uses a mold of electrically-conductive material associated with heating means involving a travelling wave field generator, is limited to low-temperature applications, particularly in the field of rubber. This field is far removed from the aims of the present invention, particularly from the point of view of the characteristics of the material worked and the thermal conditions employed.